1. Field of the Invention
The present invention relates to methods to implement a transmit diversity architecture for wireless packet data communications systems, such as those conforming to the IEEE 802.11a/g standards.
2. Description of the Related Art
Wireless communication system performance depends heavily on the radio propagation environment. For example, in a simplified two-dimensional radio propagation environment, such as a typical office environment, radio signal obstacles are represented as one-dimensional walls with certain transmission and reflection coefficients. The radio signal propagation environment between the transmitter and receiver is called a channel. Due to wall transmissions and reflections, multiple replicas of the original signal transmitted by the transmitter are received. The replicas have different amplitudes and arrival times, resulting in different channel frequency responses.
Such behavior is called frequency selective multipath fading and is typical in multipath channels. In an OFDM (orthogonal frequency division multiplexed) system, data is modulated on narrowband subcarriers. For example, IEEE 802.11a/g uses 64 narrowband subcarriers over a 20 MHz range. Because of multipath fading, each subcarrier experiences a different channel frequency response. Those subcarriers with response dips would experience lower channel gains resulting in data loss on those subcarriers.
There are different ways to mitigate the effect of multipath fading. One way is through frequency diversity where the data is spread across multiple carriers so that the deep fades on some of subcarriers can be offset by gains on other subcarriers. Another method uses spatial diversity to mitigate multipath fading. In this latter method, the transceiver uses multiple antennas (in the form of an antenna array) and RF front-ends and combines the signals from different antenna branches to mitigate multipath fading.